Visibility in Rust

pub(crate) and private fields are how a module keeps its invariants.

mod counter {
    pub struct Counter {
        count: u32,
        pub(crate) label: String,
    }

    impl Counter {
        pub fn new(label: &str) -> Self {
            Counter { count: 0, label: label.to_string() }
        }

        pub fn bump(&mut self) -> u32 {
            self.count += 1;
            self.count
        }

        pub fn count(&self) -> u32 {
            self.count
        }
    }
}

fn main() {
    let mut c = counter::Counter::new("stars");
    c.bump();
    c.bump();
    println!("{} = {}", c.label, c.count());
}

How it works

1. pub(crate) is visible in this crate but not outside it.
2. Private fields force construction through a constructor.
3. Accessors keep the internals free to change.

Keywords and builtins used here

• Counter
• Self
• String
• bump
• count
• crate
• fn
• impl
• label
• let
• main
• mod
• mut
• new
• pub
• self
• str
• struct
• u32

The run, in numbers

Lines

28

Characters to type

403

Tokens

138

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 83 seconds.

Type this snippet

Step 3 of 3 in Modules & visibility, step 3 of 12 in Modules, tests & macros.

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